1. Field of the Invention
This invention pertains to archers' compound bows. More particularly, it pertains to compound bows in which the pulleys and the rigging are arranged to eliminate torque on the limbs when the bow is at a fully drawn condition and to provide adequate spacing for the arrow fletchings to pass clear of the rigging when the bow is at or near a relaxed condition. A process for manufacturing a new pulley for use in compound shooting bows is also provided.
2. Review of the Prior Art
So far as is known, all archers' compound shooting bows use pulleys and rigging which produce substantial torque on the limbs when the bow is drawn. Archery is a rapidly expanding sport worldwide, and improvements in compound bows are continually being sought to improve their performance. This invention provides substantial performance improvements in archers' compound shooting bows.
The present invention is believed to have increased significance when used in a compound bow with intermediately pivoted limbs, such as are shown in U.S. Pat. No. 4,183,345 but the invention can be used to advantage in more conventional compound bows, such as are shown in U.S. Pat. No. 3,486,495. The new pulley may be used to advantage in compound bows having split limbs with a fork or notch at the outer ends of the limbs for mounting the new pulley directly to the end of the limb. The new pulley can also be used with bracketed limbs in which the pulley is mounted in a bracket affixed to the outer end of the limb.
Compound bows involve complex rigging of the bowstring over pulleys carried by the bow limbs. The original circular compound pulley had two parallel grooves for receiving (1) the end of the rigging cable leading to the bowstring hook (the shooting string) and (2) the end of the rigging cable running to the other limb (the bus cable). At each limb there are three connections of cables to the limb; (a) the dead end of the bus cable for the other pulley, (b) the bus cable which had its dead end connected to the other limb, and (c) the shooting string. Conventionally, the connection for (a) is to the pulley axle alongside the pulley. Bus cable (b) and shooting string (c) are connected to the pulley via the pulley grooves. When a compound bow is at a fully drawn condition, the load on each bus cable is typically on the order of two to three times the load on the shooting string. The loads on the bus cables are substantially equal. The loads of these cables are applied to each limb as the bow is drawn from a rest position to a fully drawn position. Each cable produces a torsional moment about the point where the pulley axle axis crosses the limb centerline, equal to the product of the load on the cable times the moment arm of the cable. The moment arm of a cable is defined as that distance, along the pulley axis, of the point of apparent or actual intersection of the cable with the pulley axis, measured from the limb torsional centerline or axis. The limb torsional centerline is defined as the locus of points along the limb about which torsional deflections of the limb occur. The location of the limb torsional centerline is a function of the limb geometry. For example, in a symmetrical limb, the limb torsional centerline coincides with the limb axis of symmetry.
In all known prior art arrangements, the summation of all bowstring and bus cable torsional moments about the point where the pulley axle axis crosses the limb torsional centerline has never been zero at conditions of full draw. The result has been the imposition of substantial net torque, or torsional imbalance, on the limb at full draw of the bow. This torque was at a minimum (but never zero) in the relaxed strung condition of the bow due to decreased magnitudes of the cable loads. The presence of net torque at full draw produced the problem of twisting stresses, imposed on the limbs, which are progressively additive to bending stresses experienced by the limbs, as the bow is drawn from a rest position. Thus, when flexed, the limbs of compound bows have been under significant twisting stresses, caused by torsional imbalance, which eventually leads to a degradation of limb laminal fibers and possible delamination of the limb during repeated use of the bow, and tends to shorten the useful life of the limbs of a compound bow. Typically, reinforcement of the limb tips of compound bows has been required to counteract twisting stresses on the limbs. But reinforcement of limb tips increases the inertial mass of the limbs, which disadvantageously results in slower arrow flight and a less efficient bow. For a given drawing force of a bow, an increased inertial limb mass means correspondingly less force is available for acceleration of the arrow.
Other limitations in present compound bow design are due to the presence of torsional imbalance, one of which is unsightly and appreciable angular deflection of the limb tips when the bow is at a fully drawn condition. When an arrow is shot from such a bow, the imbalance causes lateral movement of the bowstring in traveling from drawn to relaxed states as the net torque (and the corresponding angular deflection of the limb tips) is reduced. Such lateral movement produces a lateral cast (deflection) on the nock (rear end) of the arrow, causing the arrow to flex as it leaves the bowstring, and thus not fly true to the target. This effect conventionally has been partially reduced by painstaking experimental selection of arrows having the correct spine (lateral stiffness) matched to the bow as to draw length and draw weight. However, for the neophyte archer, experimentation with different (and expensive) arrows is a costly and undesirable solution.
The prior art attempted to solve the aforementioned problems by resorting to a narrow pulley rigged such that one bus cable groove was positioned on the limb torsional centerline. This operated to reduce the amount of torsional moment imbalance about the point where the pulley axis crossed the limb centerline, but not to substantially eliminate the imbalance. It was still true that the summation of angular moments about the limb centerline was not zero. Although torque was reduced, a new problem arose. The bowstring was situated so close to the bus cables that the arrow fletchings would not clear the bus cables as the arrow was shot from the bow. The prior art solved the problem of reduced and inadequate bowstring/bus cable clearance by mounting a stiff probe to the handle riser of the bow. The probe extended toward the user past the bus cables; the probe was used to displace the bus cables laterally away from the bowstring adjacent the path of movement of the bowstring. The probe, however, presented a safety hazard. Compound bows are widely used by hunters in relatively inaccessible regions where help is not readily available. If a hunter, while climbing or otherwise, should lose his balance and fall on his bow, the probe could spear him.
The use of this invention in a compound bow provides a simple and effective way to substantially eliminate torsional imbalance on the limbs at full draw of the bow, while providing adequate clearance between the bowstring and the bus cables for the arrow fletchings as the fletchings, located adjacent the rear or nock end of the arrow, move past the bus cables.
The present invention provides other advantages which are set forth in the following detailed description of preferred embodiments thereof.